Heat transfers by three mechanisms: conduction, convection, and radiation. Conduction is the molecule to molecule transfer of kinetic energy. Convection is the transfer of heat by physically moving the molecules from one place to another. Radiation is the transfer of heat through a space between two objects via electromagnetic waves.
The heat flow through solid materials is mainly by conduction. In stable heat transfer, the resistance to heat flow through solid materials can be described byQ=(T1−T2)/R  (1)
Where Q is heat flow per unit area, in w/m2 (Systeme International Unit), T1 is the higher temperature, in K, T2 is the lower temperature, in K, R is heat resistance, R-Value, in m2·K/w, defined asR=l/λ  (2)
Where l is the thickness of material which heat transfers through, in m, and λ is thermal conductivity of the material, in w/(m·K), which can be found in the prior art.
In SI unit, an R-Value such as 5.5 may be indicated as RSI5.5. In non-SI unit, as an imperial unit, R-Value uses ft2·0F·hr/Btu. The conversion between non-SI and SI is: 1 ft2·0F·hr/Btu=0.1762 m2·K/w. Thus, RSI5.5=R31.2.
The higher the R-Value of a material, the better it is to resist heat flow. R-Value of a material is measured in test laboratories. Heat flow through the layer of a material can be determined by keeping one side of the material at a constant higher temperature, for example, 90° F. (32° C.), and measuring how much supplemental energy is required to keep the other side of the material at a constant lower temperature, for example, 50° F. (10° C.). Then the R-Value can be obtained from Equation (1).
Heat transfers in liquids (air, gases, water, etc.) also by convection. In stable status, heat flux by convection can be expressed asQ=h(T1−T2)  (3)
Where h is heat transfer coefficient of convection, in w/(m2·K).
Heat convection is a combination of diffusion and bulk motion of molecules. Near the surface the fluid velocity is low, and diffusion dominates. Away from the surface, bulk motion increases the influence and dominates. Heat convection may take the form of either forced convection or natural convection. Forced convection occurs when a fluid flow is induced by an external force, such as a pump, a fan or a mixer. Natural convection is caused by buoyancy forces due to density differences caused by temperature variations in the fluid. At heating the density change in the boundary layer will cause the fluid to rise and be replaced by cooler fluid that also will heat and rise. Boiling or condensing processes are also referred as heat convection.
Heat transfer coefficient of convection h is dependent on the type of fluids, the flow properties such as velocity, viscosity and other flow and temperature dependent properties. Heat transfer coefficients of convection h can be found in the prior art.
Heat flux by radiation via a space between two objects can be expressed asQ=ϵσ(T14−T24)  (4)
Where ϵ is the emissivity of the surface, and σ is the Stefan-Boltzmann constant, 5.67037×10−8 w/(m2·K4).
Thermal radiation is the emission of electromagnetic waves from all matter that has a temperature greater than absolute zero. It represents a conversion of thermal energy into electromagnetic energy. Thermal energy results in kinetic energy in the random movements of atoms and molecules in matter. All matter with a temperature by definition is composed of particles which have kinetic energy, and which interact with each other. These atoms and molecules are composed of charged particles, protons and electrons, and kinetic interactions among matter particles result in charge-acceleration and dipole-oscillation. This results in the electrodynamic generation of coupled electric and magnetic fields, resulting in the emission of photons, radiating energy away from the body through its surface boundary. Electromagnetic radiation does not require the presence of matter to propagate and travels in the vacuum of space infinitely far if unobstructed.
The characteristics of thermal radiation depend on various properties of the surface it is emanating from, including temperature, spectral absorptivity and spectral emissive power.
Thermal radiation in a space within optical depth (for most insulation materials) is propagating via diffusion. A simple expression for conductivity of radiation can be used as followsλr=c T3/E  (5)
Where λr is thermal conductivity of radiation, in w/(m·K), c is constant related to material, radiation energy, etc., in w/(m2·K4), T is absolute temperature, in K, and E is extinction coefficient, in 1/m, can be expressed asE=1/d=ρe  (6)
Where d is the mean free path of the radiation photons, in m, ρ is density of material, in kg/m3, and e is mass specific extinction, in m2/kg.
The development of insulation materials is to make efforts to reduce heat transfer by conduction, convection and radiation. Most insulation materials have been developed before 1950s, but the extensive application of thermal insulation started after the oil crisis in 1970s. Since the oil crisis, thermal insulation of buildings has become the key issue to prevent heat loss and to improve energy efficiency. The traditional thermal insulation takes air as the best insulator. The thermal conductivity of air of 0.024 w/(m·K) sets the limit of performance for such insulation materials. Presently, only vacuum technology in combination with microporous structures can achieve the thermal conductivity of less than 0.024 w/(m·K), that is, Vacuum Insulation Panels (VIP).
In conventional insulation materials, such as fiberglass, foams, etc., three heat transfer mechanisms are required to consider. Heat transfer in gases (or air) by convection and conduction may be taken as a combination, being a thermal conductivity λg. The overall thermal conductivity λ of an insulation material can be given asλ=λg+λs+λr  (7)
Where λg is thermal conductivity of gases (or air) in pores combining heat conduction and convection, in w/(m·K), λs is thermal conductivity of the solid, in w/(m·K), and λr is thermal conductivity by radiation, in w/(m·K).
Normal insulation materials have typical overall thermal conductivity of 0.035 to 0.06 w/(m·K). To reduce overall thermal conductivity of insulation materials, VIP has been developed. VIP is to reduce λg and λr. VIP is a form of thermal insulation consisting of a nearly gas-tight enclosure surrounding a rigid core, from which the air has been evacuated. VIP consists of three main components:
(1) Walls: membrane walls or enclosures, used to prevent gases (or air) from entering the panel.
(2) Core: a panel of a rigid, highly-porous material, such as fumed silica, aerogel, perlite or fibres (glass fibres, mineral wool, etc.), to support the walls against atmospheric pressure once the air is evacuated.
(3) Getter: to collect gases (or air) leaked through the walls or offgassed from the core and wall materials.
Heat convection relies on the presence of gas molecules able to transfer heat energy by bulk movement through the insulator. Vacuum can reduce heat convection. Vacuum also greatly reduces heat conduction of gases (or air), as there are far fewer collisions between adjacent gas molecules, or between gas molecules and atoms of the core material.
Since the core material in a VIP is similar in thermal characteristics to materials used in conventional insulation, VIP therefore achieves a much lower thermal conductivity than conventional insulation materials. VIP is claimed to achieve an overall thermal conductivity of 0.004 w/(m·K) across the centre of the VIP, or an overall thermal conductivity of 0.006-0.008 w/(m·K) after allowing for thermal bridging across the VIP edges.
Core materials used in VIP are normally polyurethane (PUR) foam, expanded polystyrene (EPS) foam, extruded polystyrene (XPS) foam, silica gels, aerogels, fumed silica, glass fibres, polymer beds, perlite, etc. which are believed to be rigid to provide strength to support the walls and have lower thermal conductivity λs. Under fully evacuated, in microporous structure, heat convection and radiation in VIP are considered to be negligible.
In practical application, it is difficult to maintain vacuum in a VIP. A lot of efforts have been made to improve structure of VIP in last decades. Improvement of VIP is the development of “maintenance of vacuum”. Laminated plastic and aluminum sheets, or metal layer with a surface protection layer, can be used as wall materials for VIP, for example, in U.S. Pat. No. 4,444,821 to Young et al, in U.S. Pat. No. 4,529,638 to Yamamoto et al, and in U.S. Pat. No. 8,663,773 to Jang et al. To improve impermeability, dual walls, or two walls, or two bags are used as walls for VIP. For example, in U.S. Pat. No. 4,726,974 to Nowobilski et al, in U.S. Pat. No. 7,449,227 B2 to Echigoya et al, in U.S. Pat. No. 7,517,576 B2 to Echigoya et al, in U.S. Pat. No. 7,968,159 B2 to Feinerman, in U.S. Pat. No. 8,137,784 B2 to Veltkamp, and in U.S. Pat. No. 8,475,893 B2 to Feinerman. Multi-layers of structure of VIP was described in U.S. Pat. No. 8,383,225 B2 to Rotter. Sealing of VIP is important to maintain vacuum, various sealing methods have been developed, for example, in U.S. Pat. No. 8,281,558 B2 to Hiemeyer et al, and in U.S. Pat. No. 8,377,538 B2 to Eberhardt et al.
Getters are used to absorb or adsorb gases (or air) leaked through the walls or offgassed from the core materials. Getter materials include zeolites, activated carbon, quicklime (CaO), for example, in U.S. Pat. No. 7,838,098 B2 to Kim et al, and in U.S. Pat. No. 8,663,773 to Jang et al, and a combination of them, as well as metal-organic frameworks (MOFs), for example in U.S. Pat. No. 5,648,508A to Yaghi, and in U.S. Pat. No. 8,647,417 B2 to Eisenhardt et al.
As described above, a lot of improvements for VIP have been obtained. However, some problems still exist in VIP as high thermal resistance panels for thermal insulation, which are: (1) It is difficult to maintain vacuum for long enough for serving as conventional insulation materials. (2) Materials for walls, core and getter of VIP, and sealing VIP are expensive. (3) It is not possible to cut to sizes to fit the installations. (4) Thermal bridging across the VIP edges increases heat transfer and reduces overall heat resistance. These problems limit the practical application of VIP.
Additional improved and modified structure and mechanism to combine with the improvements addressed above are required to overcome the disadvantages in VIP to develop high resistance panels (HRP).
The following Patents and References are cited:
U.S. Pat. Nos.
4,444,821April 1984Young et al4,529,638July 1985Yamamoto et al8,663,773March 2014Jang et al4,726,974February 1988Nowobilski et al7,449,227 B2November 2008Echigoya et al7,517,576 B2April 2009Echigoya et al7,968,159 B2June 2011Feinerman8,137,784 B2March 2012Veltkamp8,475,893 B2July 2013Feinerman8,383,225 B2February 2013Rotter8,281,558 B2October 2012Hiemeyer et al8,377,538 B2February 2013Eberhardt et al7,838,098 B2November 2010Kim et al5,648,508 AJuly 1997Yaghi8,647,417 B2February 2014Eisenhardt et al